can not access via IP

Hello all
in a lan I have ispconfig installed and want to have acces via IP so someone else can get a site via VPN and IP . I raed that some people has same issues but I can not follow the solutions to get it work, cause the ispconfig server is the DNS server for that domains. I configured 4 IP's: One for ispconfig and 3 for hosting and the IP's are only used one time!! So what is the failure, when I access the website via IP and getting shared IP? And as I sayed: the IP's only used one time.